Combline filters have been used in the telecommunication industry for many decades. One of the most common types of filters for RF and microwave applications are combline filters. In particular, they are used in wireless base station applications because they offer low production cost and a relatively high unloaded quality factor (Qu). A combline filter consists of cavity resonators coupled to each other. In a conventional combline filter, each cavity has a single resonant TEM mode supported by two conductors, typically a metal bar of a square or circular cross-section is surrounded by a metallic enclosure. Cavities with more than one resonant mode can be used in dual-band and multiple-band filters. In conventional transceiver architectures, the use of different frequency bands leads to dedicated signal paths for each service requiring the use of a filter for each frequency band, which in turn results in more volume, mass, and, eventually, higher cost. To overcome these drawbacks, several transceiver architectures with dual (and multiple) band filters have been proposed for simplification of system architecture in different contexts. A dual band filter has one input and one output with two pass bands. The use of such type of filters eliminates the use of two filters and two combining networks at the input and output.
The present invention uses a new configuration of combline resonators employing multiple conductors and/or multiple dielectrics with more than one resonant mode per cavity. They are used in realizing compact filters and multiplexers with improved electric-response characteristics for modern telecommunication system applications with multiple services and several frequency bands.